Hyperthermia at temperatures above 41.degree. C., has been used sporadically as an agent for cancer therapy since the early 1900's. However, interest was not sustained because the results were inconsistent. More recently, results of studies of cell cultures in animals as well as some preliminary clinical trials, have revived the interest in the use of hyperthermia in cancer treatment. It is known that hyperthermia at temperatures above 41.degree. C. kills mammalian cells and sensitizes them to ionizing radiation. It also selectively kills and radiosensitizes cells that are relatively resistant to ionizing radiation and may eliminate or reduce recovery from sublethal and potentially lethal radiation damage. The toxicity of electron affinic compounds for oxygen deficient cells and the toxicity of several chemotherapeutic agents can also be enhanced greatly by hyperthermia. There is also evidence that hyperthermia may improve the therapeutic efficacy of radiation and chemotherapeutic agents used in therapeutic practice. Hyperthermia has been applied by fluid immersion, irrigation, regional profusion, and electromagnetic waves. Radio waves, or microwaves, appear to be the most practical and efficient means for producing localized hyperthermia. In this approach electromagnetic energy is introduced into the tissue by a field that causes oscillation of ions in the tissue or changes in the electric dipole orientation of molecules, which is then locally converted into heat.
Recently, investigations into the feasibility of using small microwave antennas or probes as a means of producing local hyperthermia in cancer therapy have employed cylindrical antennas which are inserted into the body through the esophagus or rectum, or directly into a tumor using a hypodermic needle. In most cases the antenna probe is a quarter wavelength monopole antenna with frequencies in the 500 MHz to 3 GHz range. Theoretical and experimental information indicates that a single invasive microwave antenna may be used to heat tumors of a centimeter or so in diameter to therapeutically useful levels. Multiple antennas have also been used for larger tumors. These monopole antenna probes suffer from a number of shortcomings, including poor impedance matching with the target volume of the body; high senstivity to changes in the length of penetration of the probe into the body; poor uniformity in electric field and heating patterns produced; lack of beam steering, heat sensing and visual inspection capabilities. J. W. Strohbehn, et al., "An Invasive Microwave Antenna for Locally-Induced Hyperthhermia for Cancer Therapy", Journal of Microwave Power, 14 (4), 1979, pages 339-350; D. C. deSieyes, et al., "Optimization of an Invasive Microwave Antenna for Local Hyperthermia Treatment of Cancer", Thayer School of Engineering, Dartmouth College, July 7, 1980; J. W. Strohbehn, et al., "Evaluation of an Invasive Microwave Antenna System for Heating Deep-Seated Tumors", presented at the Third International Symposium: Cancer Therapy by Hyperthermia, Drugs and Radiation, Fort Collins, Colo., June 22-26, 1980.